The present invention relates to thin film solar cells.
A) Increasing Emission Through a Potential Barrier by Altering the Surface Structure of the Potential Barrier.
In U.S. Pat. No. 6,680,214 and U.S. Pat. App. No. 2004/0206881 methods are disclosed for the induction of a suitable band gap and electron emissive properties into a substance, in which the substrate is provided with a surface structure corresponding to the interference of electron waves.
The space distribution of the probability wave associated with an elementary particle is given by:ψ*=Aexp(ikr)  (1)
where k is the wave number and r is a vector connecting initial and final locations of the particle.
FIG. 1 shows incident wave 101 Aexp(ikx) moving from left to right perpendicular to a surface dividing two domains. The surface is associated with a potential barrier.
Incident wave 101 Aexp(ikx) will mainly reflect back as reflected wave 103 βAexp(−ikx), and only a small part leaks through the surface to give transmitted wave 102 α(x)Aexp(ikx) (β≈1>>α). This is known as quantum mechanical tunneling. The elementary particle will pass the potential energy barrier with a low probability, depending on the potential energy barrier height.
In U.S. Pat. Nos. 6,531,703, 6,495,843 and 6,281,514, Tavkhelidze teaches a method for promoting the passage of elementary particles at or through a potential barrier comprising providing a potential barrier having a geometrical shape for causing de Broglie interference between said elementary particles.
Referring to FIG. 2, two domains are separated by a surface 201 having an indented shape, with height a. An incident probability wave 202 is reflected from surface 201 to give two reflected waves. Wave 203 is reflected from top of the indent and wave 204 is reflected from the bottom of the indent.
For certain values of a, the reflected probability wave equals zero meaning that the particle will not reflect back from surface 201. Leakage of the probability wave through the barrier will occur with increased probability.
Indents on the surface should have dimensions comparable to the de Broglie wavelength of an electron in order for this effect to be seen. Indents of these dimensions may be constructed on a surface by a number of means known in the art such as micro-machining. Alternatively, the indented shape may be introduced by depositing a series of islands on the surface.
For metals, this approach has a two-fold benefit. In the case that the potential barrier does not allow tunneling, providing indents on a surface of a metal creates for that metal an energy band gap due to de Broglie wave interference inside the metal. In the case that the potential barrier is of such a type that an electron can tunnel through it, providing indents on a metal surface decreases the effective potential barrier between metal and vacuum (the work function). In addition, an electron moving from vacuum into an anode electrode having an indented surface will also experience de Broglie interference, which will promote the movement of said electron into said electrode, thereby increasing the performance of the anode.
WO03083177 teaches that a metal surface can be modified with patterned indents to increase the Fermi energy level inside the metal, leading to decrease in electron work function. This effect would exist in any quantum system comprising fermions inside a potential energy box.
This approach can also be applied to semiconductors, in which case providing indents on the surface of a semiconductor modifies the size of the already present bandgap. This approach has many applications, including applications usually reserved for quantum dots.
In U.S. Pat. No. 6,117,344 methods for fabricating nano-structured surfaces having geometries in which the passage of elementary particles through a potential barrier is enhanced are described. The methods use combinations of electron beam lithography, lift-off, and rolling, imprinting or stamping processes.
WO9964642 discloses a method for fabricating nanostructures directly in a material film, preferably a metal film, deposited on a substrate. In a preferred embodiment a mold or stamp having a surface which is the topological opposite of the nanostructure to be created is pressed into a heated metal coated on a substrate. The film is cooled and the mold is removed. In another embodiment, the thin layer of metal remaining attached to the substrate is removed using bombardment with a charged particle beam.
B) Thin Film Solar Cells
Thin film solar cells made from Copper Indium Gallium Diselenide (CIGS) absorbers show great promise in achieving high conversion efficiencies, approaching 20%. The highest recorded efficiency of CIGS solar cells is by far the greatest compared with those achieved by other thin film technologies such as Cadmium Telluride (CdTe) or amorphous Silicon (a-Si).
CIGS solar cells contain several thin layers of an active material which is deposited onto an inexpensive carrier. While initially glass sheets were used as carriers, recent developments include metal foils, such as stainless steel and even polymer foils. On glass or polymers, the first layer deposited is a thin metal film serving as a back contact. Then, a semiconductor diode is formed by a combination of a CIGS (p-type) absorber with a top collector layer of CdS (n-type). Finally, a Zinc Oxide (ZnO) film and Aluminum (Al) grid are provided for the top layers and electrical contacts respectively.
FIG. 3 illustrates the basic structure of prior art CIGS solar cell 300. Shown is solar cell 300 with a layered structure comprising ZnO window layer 301, CdS collector layer 303, CIGS absorber layer 305 and metal back contact 307. Electrical connectors 309 contact the two outermost layers of solar cell 300 and are connected to electrical device 311 which is powered by the electricity generated by solar cell 300.
Photons enter solar cell 300 via ZnO window layer 301. Absorber layer 305 absorbs photons and ejects electrons thus creating pairs of electrons 313 and holes 315. Collector layer 303 is an n-type semiconductor layer in electrical contact with absorber layer 305. This layer collects electrons 313 from absorber layer 305.
The separation of electron 313-hole 315 pair is maintained by the local electrical field formed at the p-n junction between absorber layer 305 and collector layer 303. Electron 313 moves towards aluminum contacts (not shown) on the surface of ZnO window 301 while hole 315 moves towards metal back contact 307; their movement is illustrated by the arrows shown. An electrical current is thus formed, which flows through electrical connectors 309. This current can be harnessed to power electrical load 311.
The typical thin film solar cell described has several disadvantages, the major one being its low efficiency. The cell's low efficiency is due to numerous factors, several of which are outlined below:
Only those photons with energies equal to or greater than the band gap of the absorber can be absorbed to produce electricity. The majority of photons, with energies outside of this range, is either reflected or passes straight through the solar cell. The efficiency of the solar cell is thus severely limited. Attempts at controlling the band gap of the absorber by the introduction of quantum dots are currently challenged by inefficient charge carrier transport mechanisms outside the quantum dots.
Potential power is also lost when electrons and holes combine between the p-n junction and back contact, thereby generating heat rather than electricity. This mechanism is the cause of collection losses and further reduces the efficiency of thin film solar cells.
Furthermore, a natural resistance exists between the inner, sandwiched cell layers and the outer electrical contact layers in contact with the external circuit. This resistance tends to impede current flow through the external circuit, thus further diminishing the power output of the device.
Due to the low power output, operating a solar cell is relatively expensive. A modest estimate of operating costs is in the region of $3/watt. This makes the solar cell a poor competitor in the energy generation market.
From the foregoing it may be appreciated that a need has arisen for a thin film solar cell with improved efficiency, capable of responding to a broader range of photon energies and with suppressed loss mechanisms and lower operating costs.